The primary goal of this investigation is to improve our understanding of the functional role of the anterior cruciate ligament from a biomechanical viewpoint. We have hypothesized that we will be able to identify substantial and reproducible adaptive changes in loading movement patterns between normal individuals and patients with no anterior cruciate ligament (ACL). Further, it is hypothesized that the adaptation may occur over a period of time following an anterior cruciate ligament rupture and may affect clinical changes. Quantitative biomechanical information on the loading and movement of the lower extremity, myoelectric activity and time-distance information will be collected on normal subjects and patients with arthroscopically documented isolated complete tears of the anterior cruciate ligament. The ACL- subjects will be tested three times over a period of three years. The initial test will be given between six months and one year after the initial injury. Preliminary work suggests that the adaptive changes in ACL- individuals is substantial and reproducible. Information generated from this study will provide a first step in increasing our understanding of the functional adaptation associated with a missing anterior cruciate ligament. This information, combined with a wealth of available in vitro laboratory studies as well as mathematical models, will be extremely valuable in defining the functional role of the ACL as well as providing needed input into the clinical management of the patient with an injured anterior cruciate ligament.